Practical use of a solar battery as useful energy resources makes progress. The solar battery includes various types, and a silicon based solar battery, an inorganic compound based solar battery, an organic solar battery and the like are known as a typical solar battery.
In these solar batteries, a surface protective sheet is commonly provided on a surface on which sunlight falls, for the purpose of protecting the surface. A back side protective sheet (back sheet) is also provided on a surface opposite to the surface on which sunlight falls, for the purpose of protecting the solar battery cell, and it is required for the back sheet to have various excellent physical properties such as weatherability, water resistance, heat resistance, moisture barrier properties and gas barrier properties so as to suppress long-term performance deterioration of the solar battery to the minimum extent.
In order to obtain a sheet having these various physical properties, various films are used, and examples thereof include metal foils, metal plates and metal deposited films, such as aluminum, copper and steel plates; plastic films such as polypropylene, polyvinyl chloride, polyester, fluorine resin and acrylic resin films; and the like.
In order to further improve performances, a laminate obtainable by laminating these films is also used as the back sheet of the solar battery.
An example of the laminate obtained by laminating the films is shown in FIG. 1. A back sheet 10 is a laminate of plural films 11 and 12, and the films 11 and 12 are laminated by interposing an adhesive 13 therebetween.
A lamination method of films is commonly a dry lamination method, and it is required for the adhesive 13 to have sufficient adhesion to the films 11 and 12.
The back sheet 10 constitutes a solar battery module 1, together with a sealing material 20, a solar battery cell 30, and a glass plate 40 (see FIG. 3).
Since the solar battery module 1 is exposed outdoors over a long term, sufficient durability against high temperature, high humidity and sunlight is required. Particularly, when the adhesive 13 has low performances, the films 11 and 12 are peeled and thus appearance of the laminated back sheet 10 is impaired. Therefore, it is required that the adhesive for solar battery back sheets 13 does not cause peeling of the film 11 even when the adhesive is exposed over the long term.
A polyurethane is known as an example of an adhesive for solar battery back sheets. Patent Documents 1 to 3 disclose a back sheet of a solar battery produced by using a polyurethane in which a curing agent such as isocyanate is blended with a polyol for the purpose of improving various performances such as weatherability and adhesion.
In Patent Document 1, using an adhesive obtainable by combining an acrylic polymer with an isocyanate curing agent (see Patent Document 1, Tables 1 and 2) as an adhesive for solar battery back sheets, various performances of a solar battery module are improved (see Patent Document 1, Tables 3 and 4).
Patent Documents 2 and 3 disclose polyurethane resins cured by a reaction of a caprolactone based polyol with an isocyanate resin.
Patent Document 2 discloses that the above cured resin is formed on a gas barrier base material as a weatherable coat layer (see Patent Document 2, Claim 1 and [0044] to [0051]). Patent Document 2 ([0103] to [0105]) also discloses that it is possible to apply a polyurethane synthesized from a polyesterpolyol as an adhesive.
Patent Document 3 discloses that a urethane prepolymer obtainable from a caprolactone modified polyesterpolyol and a diisocyanate can be used as a material of a primer layer of a solar battery back sheet (see Patent Document 3, Claim 1 and FIG. 1). Paragraph Number 0057 of the same document discloses use of a polyurethane based adhesive.
In Patent Documents 1 to 3, a polyurethane is used so as to improve weatherability and hydrolysis resistance of a solar battery module. However, it is difficult to say that high requirements of consumers are completely satisfied. Moreover, durability required to the adhesive for solar battery back sheets is getting higher year by year, and thus higher adhesion is required to the adhesive for back sheets. Since the solar battery module is mainly used outdoors, high adhesion at high temperature is required.
Therefore, it is necessary that the adhesive for solar battery back sheets has higher adhesion (particularly, higher adhesion after aging) to a film base material, and also has sufficient adhesion even at high temperature, and has extremely high-level hydrolysis resistance capable of maintaining adhesion even though the adhesive is exposed outdoors over a long time. When the solar battery back sheet is produced by using the adhesives of Patent Documents 1 to 3, a plurality of films composing the back sheet may be peeled each other if outdoor environment is severe.
Patent Document 1: JP 2009-246360 A
Patent Document 2: WO 2010/005029 A1
Patent Document 3: JP 2011-077397 A